1. Field of Invention
The present invention relates to a light source module, and more particularly to a light source module capable of generating polarized light.
2. Related Art
In a sequence of a light ray traveling path, a structure of a liquid crystal display includes a backlight source, a first polarizer, a liquid crystal panel, and a second polarizer. The backlight source is used to generate a plane light source as uniform as possible. The plane light source is unpolarized light, and may be divided into two polarized light sources orthogonal to each other, which are defined as P light polarized in parallel with the incident plane and S light polarized perpendicular to the incident plane, commonly known as S type and P type. The first polarizer is used to select the light in a certain linear polarization direction to enter the liquid panel, and a polarization direction of the second polarizer is usually perpendicular to the first polarizer, so as to select light output by the display.
In liquid crystal display application, a light source has to be a polarized light source. The conventional way is using a polarizer to filter out polarized light of a single type, under the principle that the polarization component parallel to an absorption axis of the polarizer in the plane light source is absorbed, while the polarization component perpendicular to the absorption axis passes through the polarizer. As a polarization direction of an unpolarized light source is random, after the unpolarized light source passes through a polarizer, with a transmission loss deducted, an amount of the polarized light transmitted after filtering is less than 50% of a total amount of the unpolarized light source, that is to say, the utilization efficiency usually fails to surpass 50%. Therefore, if a degree of polarization of the backlight source may be increased, an energy loss of the light source may be effectively decreased at the polarizer, or even a polarizer (the first polarizer) may be removed.
Different technologies are used to obtain polarized light in the industry, including a reflective polarizer (DBEF, Dual Brightness Enhancement Film), a polarized light guide plate utilizing a selective total internal reflection (Selective TIR) structure, and a polarized light guide plate with a beam splitting structure plus a quarter wave plate.
The first technology (DBEF) may be found in U.S. Pat. Nos. 5,486,949, 5,828,488, 5,965,247, 6,210,785, and 6,268,961, which relate to a reflective polarizer technology. The DBEF may enable some polarized light to pass through, and reflect the orthogonal polarized light. The orthogonal polarized light that fails to be used is reflected to a backlight module for recycling use to achieve better light-emitting efficiency. In theory, the light may keep being filtered and recycled until all light rays are polarized in the same direction. A practical efficiency gain approaches 60%.
The second technology (selective TIR) may be found in U.S. Pat. Nos. 5,729,311, 7,072,544, 7,027,671, and 7,265,800. This type of architecture at least includes a layer of isotropy material as well as a layer of anisotropy material, so that a polarization selection may be achieved mainly by utilizing birefringent characteristics of the material. Such architecture is normally referred to as a selective TIR architecture.
The third technology may be found in U.S. Pat. Nos. 6,285,423, and 6,927,911, and the main structure thereof includes a prism structure placed at the bottom of a light guide plate, a layer of thin-film polarization beam splitter (PBS) between the light guide plate and the prism structure, a layer of quarter wave plate facing an end face opposite to light incident plane of the light guide plate, and a reflective surface stuck outside the quarter wave plate. The operating principle is described as follows. Light enters obliquely from the incident plane of the light guide plate. When the light ray reaches the thin-film PBS, P light penetrates the thin-film PBS and S light is reflected. After the penetrated P light is reflected by a prism structure, the P light is emitted out of the light guide plate in a small angle. The reflected S light continues its TIR propagation within the light guide plate, until the S light is transmitted through the quarter wave plate and is reflected by the reflective surface to pass through the quarter wave plate again. After the S light passes through the quarter wave plate twice, the S light is converted into a P light. Therefore, the P light may pass through the thin-film PBS and reflected out by the prism structure.
From the introductions above, it can be seen that a light source module capable of generating polarized light is very helpful in application of liquid crystal display industry.